You seem to be a victim of the common confusion between the terms weight, gravity and gravitation.
First the similarities. They involve forces and acceleration.
- Weight is the SUM of all downward and upward FORCES irrespective of their origins and points of action. Weight is essentially the supportive or opposing force required to keep something from moving (actually accelerating) upwards or downwards relative to a planetary body.
- Gravity is the sum of all downward and upward ACCELERATIONS towards or away from a planetary body. You may have heard of zero-gravity or negative g's. These are not really the absence or reversal of gravity; they are merely a set of upward forces that cancel out or exceed the usual downward forces.
Weight is simply mass x gravity WF = m.g
- Gravitation is a unique force that occurs between all matter. Gravitation has no upwardness or downwardness. Gravitational is simply a SINGLE force that exists between two objects by virtue of their mass and distance. Gravitation IS NOT a sum of forces; it is one of the forces or force mechanisms that exist alongside mechanical, electric and magnetic forces.
When a large planetary body such as the earth is involved, the major component of weight and gravity is due to the planet's gravitation; the other components may be
- upthrust- forces due to immersion in a fluid (liquid or gas),
- mechanical support,
- aerodynamic or hydrodynamic forces (drag and lift due to moving air or moving liquids),
- magnetic and electric forces due to magnetic rocks in the ground and electrical forces in the air, space, ground or roof,
- vertical centripetal forces such as those in hill climbing vehicles, aircraft, swings etc.
The main aspect of interest in weight and gravity is the vertical nature of the net force. Obviously, the term vertical is relative and that is why weight and gravity are only defined in relation to a large free body such as a planet.
Essentially, weight depends on where you are, what you are, what you are carrying and how you are moving. Weight and gravity are slightly less at the equator than at the poles because of the earth's bulge and the higher centripetal force (due to the earth's spinning).
Weight changes slightly (and negligibly) as the Moon or Sun passes overhead or underfoot and this is the cause of tides. High tides occur at roughly the time when the gravitational action of the Sun or Moon causes the weight of the ocean water to reduce.
A measurement of atmospheric pressure is essentially a measurement of the weight of the air above the barometer. Since air pressure changes, then the weight exerted on you by the air also changes (negligibly, except at high altitudes or in deep sea diving).
The gravitational pull of a trailer acting on an insect cannot be termed as either weight or gravity but it does contribute to the weight of the insect relative to the earth. Part of your weight on earth is due to the Sun pulling you upwards when overhead or downwards at midnight). The electric field of your own body is strong enough to counter the weight of dust particles and that is why they stick to your skin.
Strong winds and flood waters lift objects by causing a reduction in weight (not gravitation) through addition of upward force components. The presence of magnetic and electric rocks can and does cause noticeable changes in weight. In some places, this causes objects to roll uphill geographically although they are actually rolling downhill in terms of gravity, weight and potential energy. It is much like a steel nail been attracted uphill by a magnet.
The presence of the magnetic poles at the geographical poles may cause a slight variation in weight but the effect would be two weak to verify. Substances such as water would be repelled and weigh less while magnetic objects would way more. Magnets would weigh more or less depending on the poles that faces down towards the earth's poles. These changes in weight are probably to weak to be measured outside the margins of error due to other influences.